1. Field of the Invention
The present invention relates to a method and apparatus for providing higher order modulation in a satellite communications system that is backwards compatible with QPSK and OQPSK signal formats. More particularly, the present invention relates to a method and apparatus for reducing the power requirements for a higher order modulation signal in the satellite communications system while still maintaining the carrier phase and symbol timing synchronization for QPSK and OQPSK receivers.
2. Description of the Related Art
Satellite television users were once limited to viewing less than 50 channels, had non-High Definition Television (HDTV) viewing capability, were used for non-local station viewing, and could not receive data simultaneously with video. However, modem technology and changes in the broadcasting laws now allow satellite viewers to access over 500 channels, view HDTV and local stations, and receive data and video simultaneously. Viewers, for example, can now select channels from different categories to view music channels, history channels, news channels, etc. The large choice of channels is much greater than what is presently offered by cable and broadcast television. This has led to an increase in satellite subscribers and also to a high retention rate for current satellite subscribers.
Increasing the channel capacity to over 500 channels, providing viewers with HDTV and local station viewing capability as well as data and video capability required manufacturing new receiver boxes to receive and decode the new signals being broadcast from the satellites. The new information signals provide much more information than the conventional QPSK and OQPSK signals. DirecTV™, for example, has over 10 million subscribers. Some of these subscribers use satellite receiver boxes capable of only processing the QPSK signals. As technologies improve and operation conditions changes, it is desirable to introduce new services with more capable receiver boxes. QPSK and OPSK modulation uses two bits per symbol and are commonly depicted in a four point constellation as shown in FIG. 1A.
The new satellites for local-to-local services have spot beam transponders with substantially higher effective isotropically radiated power (“e.i.r.p.”). More information can be transmitted through these transponders with higher order modulation using new signaling formats such as 8-Phase Shift Keying (PSK) or 16-Quadrature Amplitude Modulation (QAM). 8-PSK and 16-QAM deliver 3 or 4 bits per symbol, respectively, which is greater than the 2 bits per symbol for the QPSK and OQPSK receivers. Exemplary 8-PSK and 16-QAM constellations are shown in FIGS. 1B and 1C respectively.
As discussed, the QPSK and OQPSK receivers can not demodulate conventional 8-PSK and/or 16 QAM signals. However, it is cost prohibitive for a satellite provider to retrieve the QPSK and OQPSK satellite receivers and exchange them for the new PSK and QAM receivers when not all subscribers need to receive the additional information provided by the higher modulation of the 8-PSK or 16-QAM signal format. It is also cost prohibitive to broadcast two separate signal formats using separate satellites to service the two types of receivers.
Thus, a need exists for allowing a smooth transition to a more efficient bandwidth signal format for providing additional services that are compatible with both the old and new receivers.